Scanned memory testing of multi-port memory arrays

ABSTRACT

A system for at-functional-clock-speed continuous scan array built-in self testing (ABIST) of multiport memory is disclosed. During ABIST testing, functional addressing latches from a first port are used as shadow latches for a second port&#39;s addressing latches. The arrangement reduces the amount of test-only hardware on a chip and reduces the need to write complex testing software. Higher level functions may be inserted between the shadow latches and the addressing latches to automatically provide functions such as inversions.

TECHNICAL FIELD

The present invention relates generally to the field of ASIC design andmanufacturability, and in particular, to built-in self test mechanismsfor memory.

BACKGROUND INFORMATION

Many integrated circuits facilitate defect identification using Built-InSelf Test (BIST) mechanisms. The term “BIST” can refer to testingtechniques in which parts of a circuit (chip, board, or system) are usedto test the circuit itself. BIST circuits may be formed directly on thesame chip when forming the integrated circuits and other circuitcomponents that require testing. Such BIST schemes may be used duringwafer level manufacturing test to screen out defects. Alternatively,BIST schemes may be used after each power-on to conduct self-checking ofthe circuits. The term “ABIST,” can mean “Array BIST,” or a BIST systemdesigned to test an embedded memory device. Testing multi-port memory(e.g., Processor internal Register Memory Array) may presentcomplications, such as how to fully test port interactions withoutnecessitating large amounts of extra test-only hardware. Multi-portmemory may be tested using a micro-architecture specific program such asan Architectural Verification Program (AVP). An AVP may be any softwareor firmware program that is intended to execute in a chip to verifyarchitected functions of the chip. In the case of multi-port memories,an AVP may be designed to fully verify a particular embedded memory.However, if the memory is later embedded in a different chip or has aslightly different implementation, the AVP program must be changed. Inaddition, the AVP is generally developed late in the design process,typically after the hardware is developed, and it is a complex processto test memory array cell characteristics. Since creating andmaintaining such AVP programs can be labor-intensive and burdensome, itis difficult to accomplish this late in the design process withoutcausing schedule or quality slippage.

Implementing an ABIST system may require using valuable chip area toincorporate ABIST hardware. Accordingly, to optimize an ABIST scheme, itmay be desirable to reduce the amount of “test-only” hardware needed byan ABIST system. Test-only hardware may be considered any hardwareunnecessary for normal functionality but necessary for ABIST testing.Such test-only hardware occupies valuable space on a chip and should beminimized. Optimizing an ABIST system may also require testing at speedsthat simulate functional conditions and exercise the dynamiccharacteristics of memory circuits. Additionally, scanned ABIST testingof consecutive reads, consecutive read/write, or consecutive writes of amemory typically requires additional logic configured as a set of shadowlatches for addressing.

In summary, an invention is needed that allows scanned memory ABISTtesting of multi-ported memory arrays at functional speeds, whileminimizing the amount of test-only hardware needed for ABIST testing andreducing the potential for schedule slippage.

SUMMARY OF THE INVENTION

The present invention addresses the above issues by providing mechanismsfor scanned memory testing that use functional data latches from oneport as shadow latches for another port during ABIST testing to achievefunctional speed testing of multi-ported memories.

An embodiment of the present invention is a memory array including afirst and second port. The memory array includes a first functionallatch bank. During normal (non-test) operation of the memory array, thefirst functional latch bank holds a first memory array address. Thememory array includes a second functional latch bank. During the normaloperation of the memory array, the second functional latch bank holds asecond memory array address. During a test operation, a first pluralityof latches from the first functional latch bank are interleaved to actas a plurality of shadow latches for a second plurality of latches fromthe second functional latch bank. An embodiment of the present inventionincludes a controller and an additional test-only shadow latch coupledto the controller and to a first latch of the first bank of functionallatches.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, refer to the following description taken in conjunction withthe accompanying drawings, in which:

FIG. 1A illustrates a portion of a central processing unit (CPU) thatincorporates scanned memory testing in accordance with an embodiment ofthe present invention;

FIG. 1B illustrates an ABIST controller operatively coupled to amultiport memory array;

FIG. 2A illustrates a hardware environment for testing a single-port RAMusing a shadow latch bank of test-only latches;

FIG. 2B illustrates a hardware environment for testing a multi-port RAMusing a shadow latch bank of test-only latches;

FIG. 3A illustrates a hardware environment for testing a multi-port RAMusing functional latches from port B as shadow latches for port A; and

FIG. 3B illustrates a hardware environment for testing a multi-port RAMusing functional latches from port B as shadow latches for port A withadditional circuitry for modifying the signals between the shadowlatches and functional latches.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as specific data bit lengths, address lengths, widths of datalines, and array sizes, etc. to provide a thorough understanding of thepresent invention. However, it will be obvious to those skilled in theart that the present invention may be practiced without such specificdetails. In other instances, well-known circuits have been shown inblock diagram form in order not to obscure the present invention inunnecessary detail. Some details concerning timing considerations,detection logic, specific ABIST software code and the like have beenomitted inasmuch as such details are not necessary to obtain a completeunderstanding of the present invention and are within the skills ofpersons of ordinary skill in the relevant art. Refer now to the drawingswherein depicted elements are not necessarily shown to scale and like orsimilar elements may be designated by the same reference numeral throughthe several views.

FIG. 1A illustrates major components of CPU 101, which may be part of adata processing system containing multiple CPUs. The components shown ofCPU 101 are packaged on a single semiconductor chip. CPU 101 may conductmultiple instruction issuing and hardware multithreading by concurrentlyexecuting multiple instructions and multiple threads. To supportmultiple instructions executions and hardware multithreading, processorinternal memory arrays such as floating point register 216 may havemultiple ports with multiple read ports and one write port for eachinstruction issue pipe per each thread. Accordingly, in an embodiment ofthe present invention, floating point register 216 is a multi-portmemory array that is subject to ABIST scanned testing.

Regarding the other components in FIG. 1A, CPU 101 includes instructionunit portion 200, execution unit portions 210 and 212, and storagecontrol portion 220. Instruction unit 200 obtains instructions from L1I-cache 106, decodes instructions to determine operations to perform,and resolves branch conditions to control program flow. Execution unit210 performs arithmetic and logical operations on data in registers, andloads or stores data. Storage control unit 220 accesses data in the L1data cache 221 or interfaces with memory external to the CPU whereinstructions or data may be fetched or stored.

Instruction unit 200 comprises branch unit 202, buffers 203, 204, 205,and decode/dispatch unit 206. Instructions from L1 I-cache 106 areloaded into one of the three buffers from L1 I-cache instruction bus232. Sequential buffer 203 may store 16 instructions in the currentexecution sequence. Branch buffer 205 may store 8 instructions from abranch destination. These are speculatively loaded into buffer 205before branch evaluation, in the event the branch is taken. Threadswitch buffer 204 stores 8 instructions for the inactive thread. In theevent a thread switch is required from the currently active to theinactive thread, these instructions will be immediately available.Decode/dispatch unit 206 receives the current instruction to be executedfrom one of the buffers, and decodes the instruction to determine theoperation(s) to be performed or branch conditions. Branch unit 202controls the program flow by evaluating branch conditions and refillsbuffers from L1 I-cache 106 by sending an effective address of a desiredinstruction on L1 I-cache address bus 231.

Execution unit 210 comprises S-pipe 213, M-pipe 214, R-pipe 215, and abank of general purpose registers 217. Registers 217 are divided intotwo sets, one for each thread. R-pipe 215 is a pipelined arithmetic unitfor performing a subset of integer arithmetic and logic functions forsimple integers. M-pipe 214 is a pipelined arithmetic unit forperforming a more complex larger set of arithmetic and logic functions.S-pipe 213 is a pipelined unit for performing load and store operations.Floating point unit 212 and associated floating point registers 216 areused for certain complex floating point operations that typicallyrequire multiple cycles. Similar to general purpose registers 217,floating point registers 216 are divided into two sets, one for eachthread.

Storage control unit 220 comprises memory management unit 222, L2 cachedirectory 223, L2 cache interface 224, L1 data cache (D-cache) 221, andmemory bus interface 225. L1 D-cache 221 is an on-chip cache used fordata (as opposed to instructions). L2 cache directory 223 is a directoryof the contents of CPU 101's L2 cache (not shown). L2 cache interface224 handles the transfer of data directly to and from L2 cache (notshown). Memory bus interface 225 handles the transfer of data across amemory bus (not shown), which may be to main memory (not shown) or to L2cache units (not shown) associated with other CPUs (not shown). Memorymanagement unit 222 is responsible for routing data accesses to thevarious units. For example, when S-pipe 213 processes a load command,requiring data to be loaded to a register, memory management unit mayfetch the data from L1 D-cache 221, L2 cache (not shown), or main memory(not shown). Memory management unit 222 determines where to obtain thedata and instructions. L1 D-cache 221 is directly accessible, as is theL2 cache directory 223, enabling memory management unit 222 to determinewhether the data is in either L1 D-cache 221 or the L2 cache (notshown). If the data is in neither on-chip L1 D-cache nor the L2 cache(not shown), it is fetched from memory bus (not shown) using memoryinterface 225. Similarly, if the instruction is not in L1 I-cache 106,it is fetched from the L2 cache (not shown) or the main memory throughpath 233.

Although FIG. 1A illustrates an embodiment of the present inventionimplemented within a CPU, the present invention is not limited to suchembodiments. The present invention can also be embodied in other deviceshaving logic circuitry and memory embedded on the same semiconductorchip, such as in an I/O (input/output) adapter in a data processingsystem. Additionally, embodiments of the present invention may beimplemented in conjunction with other multiport arrays, such as generalpurpose registers 217 (FIG. 1A).

FIG. 1B illustrates the interconnection of an ABIST controller 170 withfloating point registers 216, in accordance with an embodiment of thepresent invention. As shown, ABIST controller 170 is configured to testfloating point registers 216 from FIG. 1A. ABIST controller 170 receivesan ON signal 172 from an external source (not shown). In response, ABISTcontroller 170 turns ON and sends test data over test data line 168 tofloating point registers 216. The controller 170 may receive the testdata on line 176 from an external pattern generator or internal patterngenerator (not shown) within 170 may be capable of generating commontest patterns. The test data may be any of several common test patternsincluding a solid ‘1’, solid ‘0’, checkerboard, row stripe, or columnstripe. ABIST controller 170 receives response data from floating pointregisters 216 over line 166. The test data out from line 166 can beprocessed by a data comparator (not shown) in the ABIST controller 170to compare data received on line 166 with expected data values. Thecontroller 170 may use information from the comparator in ABISTcontroller 170 to determine whether the floating point registers 216pass or fail ABIST testing. Test results may be sent from the ABISTcontroller 170 to an external source (not shown) over test results line174.

When testing memory such as floating point registers 216, it may beadvantageous if ABIST controller 170 performs serial scanning of datarather than scanning the data in parallel, as in a typical ABIST scheme.Serially scanning the data using scanned ABIST testing may beadvantageous because scanned ABIST testing generally requires fewerresources such as wiring, logic space, and the like. Accordingly, it maybe easier to add new arrays to a system if ABIST testing is doneserially rather than in parallel, because adding new arrays wouldrequire fewer additional wiring and other resources.

Referring now to FIG. 2A, circuitry 250 is illustrated. Circuitry 250contains a single port RAM 252. Read and write addresses are shared andfed through RAM-address 257. FIG. 2A depicts using address/data latchbank 256 and shadow latch bank 254 for performing memory testing of RAM252. RAM 252 could correspond to a single-ported version of floatingpoint registers 216 (FIG. 1A), or any other single-port RAM. Inoperation, latch bank 256 stabilizes and holds functional addresses forsufficient time to meet timing requirements for the addresses presentedto inputs of RAM 252. Test data and addresses may be sent from ABISTcontroller 170 over line 168 to functional hold latches in latch bank256. Output data is sent back to ABIST controller through line 166.

As shown in FIG. 2A, RAM 252 is a single port RAM that may be testedusing scanned ABIST controlled by ABIST controller 170. For functionalmode, RAM-address 257 and RAM-data in 258 are fed to the latch bank 256for writing into RAM 252. Alternatively, RAM-address 257 is fed to latchbank 256 for reading RAM 252. For functional reads, RAM output iscaptured by the output latch bank 253. For scanned ABIST, shadow latchbank 254 is required. Shadow latch bank 254 is made up of test-onlyshadow latches. Shadow latch bank 254 allows the test environment totest the device under more stressful conditions, such as performing aREAD operation followed by another READ operation to two differentaddresses upon successive applications of a functional clock (not shown)running at functional clock speeds. As shown in FIG. 2A, circuitry 250requires one additional shadow latch for each functional hold latch. Theshadow latches in shadow latch bank 254 represent additional test-onlyoverhead, because they are not used for functional purposes duringoperation.

FIG. 2B illustrates a hardware environment 260 for carrying out ABISTtesting using test-only hardware latches in latch bank 264 as shadowlatches to the functional latches in latch banks 261 and 262. RAM 265depicts a multiport RAM containing port A and port B. For simplicity andclarity, components such as data ports are omitted from RAM 265 asshown, since such ports are typically understood by those of ordinaryskill in the art.

For testing port A, ABIST controller 170 sends test address data overline 168 for shadow latch bank 264, hold latch bank 261, hold latch bank262, port A, and port B. In functional mode, RAM-address A 266 andRAM-address B 267 are latched by latch banks 261 and 262, and the outputlatch bank 268 and output latch bank 269 capture RAM 265 outputs. Intesting, the outputs of RAM 265 are sent to the ABIST controller 170through scan data path 166 for testing and verifying. Latch bank 264represents the type of overhead intended to be reduced by principles ofthe present invention.

FIG. 3A illustrates representative circuitry 312 for performing ABISTtesting of multi-port memory 314 in accordance with an embodiment of thepresent invention. Memory 314 could correspond to floating pointregisters 216 from FIG. 1A. As shown, memory 314 comprises two ports;however, showing only two ports in memory 314 is not meant to limit thescope of the present invention, and principles of the invention can beextended to registers, RAM, or other memory with three or more ports. Inreality, floating point (FP) registers may be implemented withsix-reads/three-writes ports or more to accommodate multi-issues andmulti-threads. In operation, functional latch bank 320 holds RAM-addressA 340 for port A. Similarly, in operation, functional latch bank 322holds RAM-address B 350 for port B. Accordingly, functional latch banks322 and 320 hold addresses to meet the timing requirements of memory314. However, during ABIST testing, latches in functional latch bank 322are interleaved to act as shadow latches for latch bank 320. Usingfunctional hold latches in latch bank 322 as shadow latches serves tolimit the amount of test-only hardware needed for ABIST testing.

During ABIST testing of port A (FIG. 3A), ABIST controller 370 sendstest data to functional latch bank 320. Latch 324 acts as a shadow latchfor latch 326. Likewise, latch 328 acts as a shadow latch for latch 330.As shown in FIG. 3A, latch 324 is the only test-only latch needed fortesting of port A. Therefore, using the ABIST scheme shown in FIG. 3Areduces the amount of test only hardware needed when compared to theABIST scheme shown in FIG. 2B. Using port B's functional latch bank 322during testing of port A reduces the need to have dedicated shadowlatches such as those in shadow latch bank 264 (FIG. 2B). Instead ofhaving a whole bank of shadow latches such as shadow latch bank 254 or264 (FIGS. 2A and 2B), circuitry 312 utilizes shadow latch 324. Duringtesting, shadow latch 324 may provide predecessor values to thefunctional hold latch 326 during read/write operations that occur onsuccessive clock cycles. Such testing using back-to-back read and/orwrite cycles may stress a memory device and expose defects thatotherwise would go undetected. Therefore, shadow latch 324 provides theability to test RAM 314 on successive clock cycles, which isadvantageous in detecting certain defects that may exist in RAM 314. Inaddition to shadow latch 324, embodiments of the present invention mayutilize other latches (not shown). For example, a latch in the scan pathbetween different types of memory ports, such as between the address anddata ports or between the data and controls.

FIG. 3A illustrates a scheme for interleaving functional latch banksfrom one port to provide shadow latches for another port during testing.In an embodiment of the present invention, the principles shown in FIG.3A can be extended to RAMs with more than two ports. For an odd numberof ports, a similar ABIST scheme can interleave three ports asnecessary. This type of approach supports a common scannable ABISTengine (such as ABIST controller 370) that runs at functional speeds.Running at functional speeds can be helpful in observing transitiondefects that might not be detected running at lower speeds. In addition,by not requiring dedicated shadow latches for ABIST testing, embodimentsof the present invention require less logic, overhead, and labor toaccomplish ABIST testing. This results in designs that use less chiparea and power. Consequently, these designs may run faster and coolerthan other scannable ABIST solutions.

FIG. 3B illustrates a hardware environment implementing principles ofthe present invention. Like-numbered elements in FIG. 3A and FIG. 3Bcorrespond and descriptions for like-numbered items are not repeated.Compared to FIG. 3A, FIG. 3B adds circuit elements shown in circuit bank402. During ABIST testing, circuit bank 402 functions to alter thesignals between shadow latch bank 322 and functional latch bank 320. Inan embodiment of the present invention, circuit bank 402 is comprised ofan ABIST controllable function such as an inverting function; however,the components of circuit bank 402 may also be higher function logicsuch as linear feedback shift registers (LFSRs) that could automaticallyallow higher-level operations such as increasing or decreasing sequencesof numbers to latch bank 320.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations could be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A memory array comprising: a first port; a second port; a firstfunctional latch bank, wherein during a non-test operation of the memoryarray, the first functional latch bank holds a first memory arrayaddress; and a second functional latch bank, wherein during the non-testoperation of the memory array, the second functional latch bank holds asecond memory array address, wherein during a test operation, a firstplurality of latches from the first functional latch bank areinterleaved to act as shadow latches for a second plurality of latchesfrom the second functional latch bank.
 2. The memory array as recited inclaim 1, the memory array further comprising: a controller; and atest-only shadow latch coupled to the controller and to one of theplurality of latches from the first functional latch bank.
 3. The memoryarray as recited in claim 2, the memory array further comprising: afirst input to the controller, wherein the input is operatively coupledto a first output of a fourth latch bank, wherein a third latch bank isoperatively coupled to the first functional latch bank.
 4. The memoryarray as recited in claim 3, the memory array further comprising: asecond output to the third latch bank, wherein the second output isoperatively coupled to a second input of the fourth latch bank.
 5. Thememory array as recited in claim 1, wherein the test operation occurs ata first clock speed substantially equal to a second clock speed of thenon-test operation.
 6. The memory array as recited in claim 4, thememory array further comprising: a circuit bank operatively coupled tothe first functional latch bank and the second functional latch bank foraltering a plurality of signals during the test operation.
 7. The memoryarray as recited in claim 6, wherein the circuit bank includes aplurality of inverters.
 8. The memory array as recited in claim 6,wherein the circuit bank includes a plurality of linear feedback shiftregisters.
 9. The memory array as recited in claim 2, wherein the memoryarray is a floating point register.
 10. The memory array as recited inclaim 2, wherein the memory array is random access memory (RAM). 11-20.(canceled)